1. Field of the Invention
This invention relates in general to earth boring drill bits, and in particular to the cutter bearing and seal arrangement, and to the insert arrangement of a large diameter shaft bit.
2. Description of the Prior Art
Drill bits for large diameter shafts normally have a cutter support plate that is connected to a string of drill pipe for rotation. A number of cutter assemblies are rotatably secured to the cutter support plate to disintegrate the earth as the cutter support plate is rotated. Drilling may be downward, or upward by pulling the bit through a pilot hole, as in raise drilling.
Each cutter assembly includes an axle for securing to a cutter mount attached to the cutter support plate. A cutter sleeve is mounted on the axle by roller bearings, with the ends of the axle extending beyond each side of the sleeve. A seal is located on each side of the sleeve between the axle and the sleeve to prevent grit from entering the bearings. Typical types are shown in U.S. Pat. Nos. 3,612,196 and 3,216,513. In these patents and in all other types known to applicant, the seals are equal or slightly larger in diameter than the bearings.
One disadvantage of having large diameter seals is that the surface velocity between the moving parts is higher and generates more heat than would occur if the seal were a smaller diameter. Any single point on the seal moving surface will travel a greater distance and therefore the life of the seal will be reduced. Another disadvantage is that since the seals are located at the sides of the bearings, the width of the cutter cannot be reduced significantly without using smaller width bearings. In certain cases, a smaller width cutter is desirable.
Another common feature in drill bits for shaft boring and for earth boring in general, is the tendency to "track." "Tracking" is a condition which results when a cutter tooth repeatedly engages a previously made depression in a borehole bottom or face. As a result, a crest of rock may be generated on the face, which may lead to disadvantages such as erosion of the cutter shell or premature tooth disintegration. "Tooth" is used herein to include both tungsten carbide or other hard metal inserts secured in holes in the cutter exterior, and also steel teeth formed in the cutter exterior. As indicated in my prior U.S. Pat. No. 3,726,350, tracking is more difficult to avoid in types of cutters that approach true rolling. And true rolling contact is often advantageous to cutter life in rock drilling, especially in bits that utilize hard metal inserts.
One prior art method to avoid tracking is to dimension the cutter so that the ratio of the circumference described on the borehole face by a row of cutter teeth to the circumference of that row on the cutter does not equal an "integer." "Integer" is a whole (not fractional or mixed) number. Teeth arrangements to prevent tracking have also been utilized, such as shown in my above mentioned patent. Yet the problem still exists. For example, laboratory tests have indicated that a cutter may slip slightly and fall back into a previous depression. If the inserts are evenly spaced about the cutter, this slippage at one point may place the rest of the inserts back into the old pattern. Certain proposals have groups of inserts within a row separated from other groups. However, as far as known to applicant, the distance between the center lines of adjacent teeth in a circumferential row is uniform within all groups of inserts in the row.